US3536885A - Plasma torch assemblies - Google Patents

Description

ZLE U LQQ Oct. 27, 1970 I P. MITCHELL 3,535,885

PLASMA TORCH ASSEMBLIES Filed Oct. 20, 1966 3 Sheets-Sheet 1 FIG! Oct. 27, 1970 P. MITCHELL PLASMA TORCH ASSEMBLIES 3 Sheets-Sheet 2 Filed Oct. 20, 1966 FIG FIG?

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Oct. 27, 1970 P. MITCHELL PLASMA TORCH ASSEMBLIES 3 Sheets-Sheet 5 Filed Oct. 20. 1966 FIGEB FIGS) United States Patent 3,536,885 PLASMA TORCH ASSEMBLIES Peter Mitchell, Birmingham, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Oct. 20, 1966, Ser. No. 588,172 Claims priority, application Great Britain, Oct. 25, 1965, 45,096/ 65 Int. Cl. B23k 9/00 US. Cl. 219-121 18 Claims ABSTRACT OF THE DISCLOSURE The operation of a plasma torch as an alternating current supply at power frequency while obtaining the benefit of low electrode wear normally associated with DC. operated torches by arranging that an arc is formed, and plasma forming gas converted to plasma, only in similar halfcycles. This is achieved by passing a pilot plasma into the plasma forming chamber and by controlling the composition and rate of flow of the plasma forming gas relative to the pilot plasma such that the torch operates as a rectifier. A plurality of torches can be interconnected to operate on either similar or opposite half-cycles of the supply voltages.

Plasma torches which are so constructed as to operate from either a DC. source or a high frequency A.C. source are known but it is inconvenient to have to supply these special sources of electrical energy and it is an object of the present invention to provide a plasma torch which can be operated from an alternating voltage source of mains frequency.

According to the present invention a plasma torch comprises having two .electrodes which provide inlet and outlet ports respectively for the chamber, means for passing plasma-forming gas into the chamber and between the electrodes and means for generating a pilot plasma which is passed through the chamber by way of the inlet and outlet ports whereby a unidirectional current flows between the electrodes by way of said pilot plasma when a power frequency alternating voltage is applied therebetween.

One of the advantages of such torches is that the electrodes have the low-wear characteristics usually associated with plasma torches which are operated from a DO.

source.

The pilot plasma projected into the chamber through the inlet port electrically couples the two electrodes allowing a current to flow between them from the alternating source. The electrode which provides the inlet port is heated, when in use, by the pilot plasma passing therethrough and the relatively cold plasma forming gas flowing into the chamber cools the electrode which provides the outlet port to a greater extent than it cools the other electrode. Furthermore by arranging for the plasma forming gas conveniently nitrogen which is directed into the chamber to have a much greater electrical breakdown potential than that of the gas, conveniently argon, from which the pilot plasma is formed, it can be arranged for the breakdown voltage in the direction from the outlet port electrode to the inlet port electrode to be much higher than the breakdown voltage in the reverse direction. These factors enable a unidirectional current to flow between the electrodes. The direction of electron flow is from the electrode which provides the inlet port to the other electrode i.e. current flows during the half cycle of the applied voltage when the electrode which provides the outlet port is positive with respect to the electrode which provides the inlet port.

The pilot plasma which is passed through the chamber may be generated by any convenient means from a DC.

3,536,885 Patented Oct. 27, 1970 ice reverse polarity DC). or AC. or half wave A.C. supply. When a half wave A.C. supply is employed it is essential that the plasma is generated at the periods of time when conduction is required between the electrodes of the torch. The pilot plasma is generated between a pair of electrodes which may be completely separate from the electrodes of the torch but alternatively one of the elec trodes between which the pilot plasma is generated may be an axial extension of, or the reverse end of, the electrode which provides the inlet port of the chamber.

In order that the invention may be more readily understood, it will now be described, by way of example with reference to the accompanying drawings in which:

FIG. 1 is a sectional side elevation of a plasma torch in accordance with one embodiment of the invention, showing the power supplies thereto;

FIGS. 2 and 3 show in diagrammatic form alternative electrical connections to the torch when used to heat an electrically conductive material by transferred flame operation;

FIGS. 4 and 5 show in diagrammatic form alternative electrical connections to an assembly comprising a pair of plasma torches;

FIGS. 6 and 7 show in diagrammatic form alternative electrical connections to a pair of plasma torches to bring about transferred flame operation;

FIG. 8 shows in diagrammatic form the electrical connections to a device employing four plasma torches to bring about transferred flame operation thereof, from a single phase supply; and

FIG. 9 shows in diagrammatic form the electrical connections to a device employing six plasma torches to bring about transferred flame operation from a three-phase electrical supply.

Referring particularly to FIG. 1, a plasma torch 1 comprises a pair of conductive electrodes 2 and 3 which are separated by an annular body of electrically insulating material 4 and which together define a generally cylindrical chamber 5. The electrodes 2 and 3 each have an opening therein which openings are in axial alignment and provide an inlet port 6 and an outlet port 7 respectively to the chamber. Electrode 2 is provided with a hollow cylindrical part 8 manufactured from a refractory metal i.e. tungsten and which projects into the chamber 5 with the bore of the part in alignment with the opening in the electrodes. Electrode 3 is of refractory metal or copper and both of the electrodes have provision for liquid cooling in the vicinity of the opening therein.

A gas inlet port 9 is provided through the wall of the electrode 3 into the chamber and the port is inclined almost tangentially to the inner surface of the wall of the cylindrical chamber so that plasma forming gas injected under pressure through the port into the chamber swirls round the chamber in a vortex about the axis of the inlet and outlet ports 6 and'7.

Electrode 2 is electrically insulated from a further electrode 10 by an annular body 11 of insulating material. The electrodes 2 and 10 and the body 11 define a further chamber 12 which is of generally cylindrical form and in communication with the chamber 5 by way of the port 6. A rodlike extension 13 projects from the electrode 10 into the chamber 11 in axial alignment with the ports 6 and 7. The extension 13 may be of tungsten and with the electrode 2 constitutes a pilot plasma generator. Plasma forming gas is injected under pressure into the chamber through an inlet port 14 so as to swirl around the chamber. The electrode 10 may be liquid cooled.

To operate the assembly plasma forming gas, which conveniently may be argon, is injected into chamber 12 through the port 14 and a unidirectional voltage conveniently from a rectifier 15 fed from a transformer 16 is applied to terminals on the electrodes 10 and 2 with electrode 10 negative with respect to electrode 2. An electric arc is set up between the electrodes and the plasma forming gas passing through the arc is converted to a plasma which streams through the opening in electrode 2 into the chamber 5. A mains frequency alternating voltage from a transformer 17 is applied between terminals on electrodes 2 and 3 and the pilot plasma provides a conductive path between these electrodes. An arc extends between the outer tip of extension part 8 and the adjacent surface of the electrode 3, and plasma forming gas injected into the chamber through port 9 constricts the arc and forces it part way into the outlet port 7. The plasma forming gas which conveniently may be nitrogen passes through the arc and is converted into a plasma which reinforces the pilot plasma leaving the chamber 5 through the outlet port 7. By suitably adjusting the spacing between the electrodes 2 and 3 and the composition and rate of flow of the plasma forming gases injected into the chambers 5 and 12, the torch can be made to operate in a self rectifying manner. Although the pilot plasma may flow continuously into the chamber 5 an arc is only formed between electrodes 2 and 3 during the half cycles of the supply applied between these electrodes when electrode 3 is positive with respect to electrode 2, i.e. electrode 2 acts as the cathode and electrode 3 as the anode. During the reverse half cycles there is no current flow between the electrodes.

The plasma produced by the torch shown in FIG. 1 is usually used as a heating source but by modifying the electrical connections to the torch as shown in FIGS. 2 and 3 the plasma leaving the torch can be used as a conductive column between the electrodes of the torch and an electrically conductive work-piece or charge. In FIGS. 2-9 the plasma torch shown in FIG. 1 is indicated diagrammatically by the three electrodes 13, 2 and 3 and the electrical supply for producing the pilot plasma is not shown.

Referring to FIG. 2, an alternating mains frequency electrical supply from transformer 17 is connected between the electrodes 2 and 3 and a further alternating supply of the same mains frequency from a transformer 21 is connected between the electrode 2 and a conductive charge 22 in a furnace indicated by reference numeral 23. The polarity of the two electrical supplies are arranged so that the charge is positive 'with respect to the electrode 2 at the same time that electrode 3 is positive with respect to electrode 2 and during the half cycles when this occurs the plasma leaving the torch is used to pass current into the charge. The current flow in the two electrical circuits is shown by the full arrowed lines. If as shown in FIG. 3, the supply from the transformer 21 is connected between the electrode 3 and the charge in the furnace with the polarity of the windings of transformers 17 and 21 arranged such that the charge is negative with respect to electrode 3 when electrode 2 is negative with respect to electrode 3 then half cycles of electrical energy flow between the electrodes 2 and 3 and 1 half wave current flows through the plasma from the charge to the electrode 3. It will be seen that to draw a full-wave current from the supply connected between the electrodes 2 and 3 it is necessary to employ two torches and operate them alternately. The electrical connection for such an assembly is shown in FIG. 4. Two torches 40 and 41 are connected together electrically with the electrode 2 of torch 40 connected to the electrode 3 of torch 41 and the electrode 2 of torch 41 and the elec trode 3 of torch 40 are similarly connected together. The electrical supply from a transformer 42 is connected between the electrodes 2 and 3 of torch 41. During the half cycles of the supply when the electrode 3 of torch 41 is positive with respect to the electrode 2 of the torch, electrons flow between them in the direction indicated by the full arrowed line and current is drawn from the supply. During the alternative half cycles when the electrode 2 of this torch is positive with respect to the electrode 3 of the torch, no electrons flow between them. However during these latter half cycles the electrode 3 of torch 40 is positive with respect to its electrode 2 and half cycles of current are drawn from the supply and flow bet-ween these electrodes in the direction indicated by the broken arrowed line.

FIG. 5 shows an alternative arrangement in which the electrode 2 of two torches 50 and 51 are connected to the respective outer ends of a centre tapped secondary winding of a transformer 52 and a centre tap is connected to the electrode 3 of both torches. During the half cycles of the supply when the electrode 3 of torch 50 is positive with respect to the electrode 2 thereof current flows therebetween in the direction shown by the full arrowed line. During the alternate half cycles of the supply when the torch 50 is not conducting, torch 51 conducts with the current flowing in the direction indicated by the broken line.

The arrangement illustrated in FIG. 6 is a combination of the features of the assemblies shown in FIGS. 3 and 4 to enable a continuous transferred arc to a conductive charge 22 to be made. The same result is obtained by the assembly shown in FIG. 7 which combines the feature of the torch and assembly shown in FIGS. 2 and 5 respectively.

FIG. 8 illustrates a device for transferred-arc operation but without the necessity of applying an electrical connection direct to the charge. Two assemblies 81 and 82 each of the type illustrated in FIG. 4 and each consisting of two torches a and b have a further mains frequency electrical supply from a transformer 83 applied between the electrode 3 of torch 81b and the electrode 3 of torch 82a. The streams of plasma from the torches to the conductive charge 22 serve as conductive columns between the torches and the charge and a full wave current is taken from the secondary winding of transformer 83. During one set of alternate half cycles the electron flow is as indicated by full arrowed lines and during the other set of alternate half cycles the electron flow is as indicated by broken arrowed lines.

FIG. 9 shows a device by which three plasma torch assemblies 90, 91, and 92 each consisting of two torches can be operated with transferred are from a three-phase supply. The two torches in each assembly have their noncorresponding main electrodes connected together as described in connection with FIG. 4 and the electrode 2 of the corresponding torch of each assembly is connected to a separate phase of the three phase secondary windings of a three phase transformer 93. The full arrowed lines indicate the electron flow during one half cycle of one phase of the supply and the broken arrowed lines indicate the direction of electron flow during the reverse half of that cycle. Although the phases of the transformer 93 are shown connected in star, a delta connection can be employed provided that the phase of the voltage between the main electrodes is maintained the same as the corresponding transfer voltage. There is no direct electrical connection between the conductive charge 22 and any of the torches or the various electrical supplies.

What I claim is:

1. A method of operating a plasma torch from a source of alternating voltage at power frequency including the steps of applying said voltage without prior rectification between a pair of electrodes of the torch, continuously passing a pilot plasma between said electrode, passing a plasma forming gas between said electrodes with the composition and rate of bow of the plasma forming gas selected in relation to the electrodes to cause the torch to operate as a rectifier, such that arcs are struck between the said electrodes only during similar half cycles of said voltage but not during the opposite half-cycles, to convert said plasma forming gas to plasma only during said similar half-cycles.

2. A method of operating a plasma torch from a source of alternating voltage at power frequency, as claimed in claim 1 in which said electrodes provide an inlet and an outlet port respectively, of a chamber into which said pilot plasma and said plasma forming gas are directed and holding said electrode which provides the outlet port at a lower operating temperature than that providing the inlet port whereby said arcs are struck only during the half cycles of the alternating voltage when said electrode providing the outlet port is positive with respect to the other electrode.

3. A method of operating a plasma torch as claimed in claim 2, in which said plasma forming gas introduced into the chamber serves to cool the electrode providing the outlet port to a greater extent than it cools the electrode providing the inlet port.

4. A method of operating a plasma torch from a source of alternating voltage at power frequency as claimed in claim 1 in which said electrodes provide an inlet port and an outlet port respectively of a chamber, said pilot plasma is directed into said chamber through said inlet port, said pilot plasma is formed from a gas having a much lower electrical breakdown potential than that of the plasma forming gas passed between the electrodes and said arcs are struck only during the half cycles of the alternating voltage when said electrode providing the outlet port is positive with respect to the other electrode.

57 A method of operating a plasma torch from a source of alternating voltage at power frequency as claimed in claim 1 in which one of said electrodes provides an inlet port and the other electrode provides an outlet port of a chamber, said pilot plasma is directed into said chamber through said inlet port, said pilot plasma is formed from argon, said plasma forming gas which is passed between said electrodes is nitrogen and said arcs are struck only during the half cycles of the alternating voltage when said electrode providing the outlet port is positive with respect to the other electrode.

6. A method of operating a plasma torch as claimed in claim 1 to heat an electrically conductive material in which the plasma generated in said torch is directed onto said conductive material and a further source of alternating voltage at power frequency is connected between said conductive material and one of the electrodes of the torch such that half waves of current flow between said electrode and the conductive material.

7. The method of claim 1, wherein said plasma forming gas is introduced in an amount sufficient to maintain the second electrode at a temperature substantially below that of the first electrode.

8. The method of claim 1, wherein the plasma forming gas passed between the said electrodes has a higher electrical breakdown potential than the plasma forming gas used to form the said pilot plasma.

9. A plasma torch assembly comprising two similar plasma torches each comprising a pilot plasma chamber, a main plasma forming chamber, first and second spaced apart electrodes which are apertured to provide an inlet and an outlet port respectively for said main plasma forming chamber and which are electrically insulated from one another, and first and second terminal means in electrical contact one with each of the electrodes for enabling an alternating voltage to be applied thereto, means for introducing a gas having a low electrical breakdown potential into the pilot plasma chamber for the generation of a pilot plasma introduceable into the main plasma forming chamber through the inlet port, means for introducing plasma forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamber of each said torch, such that the two torches are operable as rectifiers to convert the plasma forming gas to plasma in alternate half cycles of said voltage, and electrical connections between the first electrode of each torch and the second electrode of the other torch.

10. A plasma torch assembly comprising two similar plasma torches each having a pilot plasma chamber, a main plasma forming chamber, first and second spaced apart electrodes which are apertured to provide an inlet and an outlet port respectively for said main plasma forming chamber, and which are electrically insulated from one another, means for introducing a gas having a low electrical breakdown potential into the pilot chamber for the generation of a pilot plasma introduceable into the main plasma forming chamber through the inlet port, means for introducing a plasma-forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamber of each torch, electrical connections between the first electrodes of each torch and the second electrode of the other torch, and terminal means for enabling an alternating voltage at power frequency but of different polarity to be applied to the electrodes of the two torches to cause each torch to operate as a half-wave rectifier and its plasma forming gas to be converted to plasma only during the half-cycles of said voltage in which the second electrode is positive with respect to the first electrode.

11. A plasma torch assembly according to claim 10, wherein each of the torches is disposed so as to direct the plasma generated therein onto said conductive material through said outlet port and wherein there are included means for applying an alternating voltage at power frequency between said conductive material and the second electrode of one of the torches.

12. A plasma torch assembly comprising tWo similar plasma torches each comprising a pilot plasma chamber, a main plasma forming chamber, first and second spaced apart electrodes which are apertured to provide an inlet and an outlet port respectively for the main plasma forming chamber, and which are electrically insulated from one another, first and second terminal means in electrical contact one with each of the electrodes, means for introducing a gas having a low electrical breakdown potential into the pilot chamber for the generation of a pilot plasma introduceable into the main plasma forming chamber through said inlet port, means for introducing plasma forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamber, and a transformer having a tapped secondary winding opposite ends of which are connected each to one of a pair of similar electrodes of the torches and the tapping of which is connected to the other pair of similar electrodes of the two torches, means for applying an alternating voltage at power frequency to the primary winding of the transformer to cause electric arcs to be struck between the electrodes of each torch in turn during the half-cycles in which the electrode of the torch which is connected to the tapping is positive with respect to the other electrode of the torch, and thereby to convert the respective plasma forming gas to plasma during said halfcycles.

13. A plasma torch assembly for heating an electrically conductive material comprising two similar plasma torches each comprising a pilot plasma chamber, a main plasma forming chamber, first and second spaced apart electrodes which are apertured to provide an inlet and an outlet port respectively for the main lasma forming chamber, and which are electrically insulated from one another, first and second terminal means in electrical contact one with each of the electrodes, means for introducing a gas having a low electrical breakdown potential into the pilot chamber for the generation of a pilot plasma introduceable into the main plasma forming chamber through said inlet port, means for introducing plasma forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamber, and a transformer having a primary winding arranged to be connected to a source of alternating voltage at power frequency and secondary windings connected so as to apply between the electrodes of the two torches alternating voltages of opposite polarity such that the electrodes of the two torches act as a rectifier to convert the respective plasma forming gases to plasma only during different alternate half-cycles, a further transformer having a primary winding arranged to be connected to a source of alternating voltage at power frequency and a tapped secondary winding opposite ends of which are connected each to one of a pair of similar electrodes of the two torches and the tapping of which is connectable to the said conductive material.

14. Apparatus for heating electrically conductive material comprising a pair of plasma torch assemblies, each of which assemblies comprises two similar plasma torches each having a pilot plasma chamber, a main plasma-forming chamber, first and second spaced apart electrodes apertured to provide an inlet and an outlet port respectively for said chamber, and electrically insulated from one another, first and second terminals connected to the first and second electrodes, means for introducing a gas having a low electrical breakdown potential into the pilot chamber for the generation of a pilot plasma introduceable into the main plasma forming chamber through said inlet port, means for introducing plasma forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamher, the terminals of one of the torches of each assembly being connected to a respective source of alternating voltage at power frequency, electrical connections being provided between the first electrode of one torch of each assembly to the second electrode of the other torch of the assembly such that the two torches are operable to convert the respective plasma forming gases to plasma in alternate half-cycles of the applied voltage, and a transformer having its secondary winding connected between similar electrodes of two torches one from each assembly, and its primary winding connected to a source of alternating voltage at power frequency.

15. Apparatus for heating electrically conductive material comprising three plasma torch assemblies each of which comprises two similar plasma torches each having first and second spaced apart electrodes electrically insulated from one another, first and second terminals connected to the first and second electrodes, means for introducing a gas having a low electrical breakdown potential into the pilot chamber for the generation of a pilot plasma introduceable into the main plasma forming chamher through said inlet port, means for introducing plasma forming gas of higher electrical breakdown potential under pressure between the electrodes in said main plasma forming chamber, and electrical connections being provided between the first electrodes of each torch of each assembly, and its second electrode of the other torch of the assembly such that the two torches are operable to convert the respective plasma forming gases to plasma in alternate half-cycles of the applied voltage, and corresponding electrodes from each of the three assemblies being connected to respective secondary windings of a three-phase transformer, the primary winding of which is connected to a three-phase alternating voltage source at power frequency.

16. A plasma torch comprising means defining a chamber, two spaced apart electrodes which provide inlet and outlet ports for the chamber, said electrodes including first and second electrodes which are electrically insulated from one another, means for introducing lasma forming gas under pressure into the chamber between said first and second electrodes so that the gas flows between said electrodes, means for generating a pilot plasma which continuously enters the said chamber via said inlet port, terminal means for enabling and alternating voltage at power frequency to be applied between the two electrodes, and means for operating the torch such that the torch opcrates as a rectifier and arcs are formed and the plasma forming gas converted to plasma only during similar half-cycles of said alternating voltage.

17. A plasma torch according to claim 16 wherein the means for operating the torch so that the torch operates as a rectifier comprises means for introducing said plasma forming gas such that its electrical breakdown potential is greater than the plasma forming gas used to form the said pilot plasma.

18. A plasma torch according to claim 16, wherein the means for operating the torch so that the torch operates as a rectifier comprises means for introducing said plasma forming gas under pressure between the electrodes in the main plasma-forming chamber to be expelled from the outlet port, in an amount sufficient to maintain the second electrode at a temperature substantially below that of the first electrode.

References Cited UNITED STATES PATENTS 3,248,513 4/1966 Sunnen 219121 3,373,306 3/1968 Karlovitz 219- 2,756,311 7/1956 Persson et al 219- RALPH F. STAUBLY, Primary Examiner J. G. SMITH, Assistant Examiner US. Cl. X.R. 21975

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